As well known, tidal power plants are arranged to convert into electricity the energy of tides. To this purpose, in case of a tidal lagoon, turbine housing may be arranged between the sea and the lagoon basin. The turbine housing may include a bulb runner unit comprising a plurality of blades fit thereon which are moved by the flow of water. The bulb runner is integral to a rotating shaft which cooperates with an electricity generator.
Depending on the tidal level, when the water level (also known as “head”) of the sea rises with respect to the level of the lagoon, water can start flowing through the turbine to produce energy. Similarly, as the sea level starts to fall, a tidal head can be created by holding water back in the lagoon until a sufficient head is formed. Thus the process can be reversed and the water flows in the opposite direction from the lagoon to the sea through the turbine. In this way the generation of electricity is maximised, as it occurs with the flow of water in both senses.
However, the blades fit to the bulb unit usually have a fixed direction with respect to the flow of water. The consequence of such fixed arrangement usually ensures an acceptable efficiency when the runner is operating in the direct mode, which is when the water flows from the lagoon to the sea, but at the same time a significant decrease of efficiency is experienced when operating in the reverse mode, since the same inclination of the blades is maintained in both operating modes.